Parahorror

One would almost never their actual forms, Parahorrors are actually the result of a magical parasite taking over a corpse. Old scrolls and tomes from Aetza tell of these parasites and why they choose primarily humanoid hosts. Both humanoids and these parasites share similar magical energy. While a parasite might choose a monster corpse to take over, they would have to be extremely lucky and/or quick to take over the host before it dies completely and the body turns into dust. This is because taking over a host takes some time and some transmutation of the body itself. As a result, humanoid bodies are best since they don't disintegrate like monster bodies do. It's said this is a big reason why Aetza usually cremates their dead, rather than bury them.
Fighting a Parahorror should be avoided, though all of them have a weakness, and that is themselves. If one manages to rip out or somehow dislodge the parasite from its host, it will die instantly. Usually, the parasite is located somewhere along the spinal cord of the host. Getting to it usually proves difficult however thanks to the many things a Parahorror can do. Beware of its ability to transmute and renegerate quickly.